Acid Base Abnormalities

Question 1

How to interpret ABG

Answer 1

• 1) Look at pH:
• <7.35 is acidosis
• >7.45 is alkalosis

• 2) Look at CO2:
◦ CO2 is an acidic gas
◦ If CO2 low and pH high, then respiratory alkalosis
◦ If CO2 high and pH low, then respiratory acidosis

• 3) Look at HCO3:
◦ HCO3 is alkaline
◦ If HCO3 low and pH low, then metabolic acidosis
◦ If HCO3 high and pH high, then metabolic alkalosis

• 4) Any compensation:
‣ Respiratory Acidosis->high bicarbonate (but still acidotic pH)= partial metabolic compensation
‣ Respiratory Alkalosis->low bicarbonate (but still alkalotic pH)= partial metabolic compensation

				‣ Metabolic Acidosis-> low CO2 (but still acidotic pH)= partial respiratory compensation 
				‣ Metabolic Alkalosis-> high CO2 (but still alkalotic pH)= partial respiratory compensation

Question 2

Causes of Metabolic Acidosis

Answer 2

• Caused by REDUCTION in PLASMA BICARBONATE levels and hence a DROP in pH
• Caused by 2 mechanisms:
• Increased production or reduced excretion of acid:
◦ This leads to Metabolic Acidosis with an INCREASED ANION GAP:
◦ Lactic acid (shock, infection, tissue ischaemia)
◦ Urate (renal failure)
◦ Ketones (diabetic ketoacidosis, alcohol)
◦ Drugs/Toxins (salicylate overdose- direct acid effect and renal failure cause this)

					• Loss of bicarbonate or ingestion of H+:
						◦ This leads to Metabolic acidosis with a NORMAL ANION GAP:
						◦ GI bicarbonate loss (DIARRHOEA)
						◦ Renal tubular acidosis
						◦ Drugs (acetazolamide)
						◦ Addison’s disease

Anion gap: (N+ + K+) - (Cl- + HC03-)

Question 3

Metabolic Alkalosis causes

Answer 3

• Caused by a RISE in BICARBONATE levels and hence a RISE in pH
• Usually due to a loss of H+ ions or a gain of bicarbonate ions
• Causes:
‣ Persistent vomiting: loss of H+
‣ Diuretics: loss of K+
‣ Burns
‣ Hypokalaemia

Question 4

Causes of respiratory acidosis

Answer 4

• Caused by RISE in CO2 and hence a DROP in pH
• Usually due to alveolar hypoventilation (type 2 respiratory failure), make sure to look at PaO2
• Causes:
‣ COPD
‣ Exhaustion in severe asthma, pneumonia and pulmonary oedema (they tire out and begin to retain CO2, risk of respiratory arrest)
‣ Neuromuscular or physical cause of hypoventilation
‣ Sedative drugs: benzodiazepines and opiate overdose (respiratory depression)

Question 5

Respiratory alkalosis

Answer 5

• Caused by DROP in CO2 and hence a RISE in pH
• Usually due to hyperventilation (leading to loss of CO2)
• Causes:
‣ Psychogenic: anxiety leading to hyperventilation
‣ Hypoxia: causing hyperventilation (due to PE, high altitude)
‣ Early salicylate poisoning: due to stimulation of respiratory centres
‣ CNS: stroke, SAH, encephalitis
‣ Pregnancy
‣ Asthma attack (earlier stages)-hypoxia